Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics, separated from such raw materials as petroleum naphthas, particularly reformates, usually be selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point.degree. F. Point.degree. F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethyl benzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
In turn, calculated thermodynamic equilibra for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt.% Ethyl benzene 8.5 Wt.% para xylene 22.0 Wt.% meta xylene 48.0 Wt.% ortho xylene 21.5 TOTAL 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt. % ortho-xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt. %.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.
Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
______________________________________ Process Conditions ______________________________________ Reactor Pressure 175 to 225 PSIG Reactor Inlet Temper- ature Range 830-900.degree. F. Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr. Number of Reactors, Downflow 1 Catalyst Bed Depth, Feet 11 to 15 Catalyst Density, Lb/Cu. Ft. 38 Recycle Circulation, Mols Hydrogen/Mol Hydrocarbon Feed 7.0 to 14.0 Maximum Catalyst Pressure Drop, PSI 20 ______________________________________
It will be seen that the system is adapted to produce maximum quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethyl benzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining has been unique in its ability to convert ethyl benzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C.sub.8 aromatic fractions. As will be seen from the table of properties above, the boiling point of ethyl benzene is very close to those of p- and m-xylene. Complete removal of ethyl benzene from the charge is impractical. The usual expedient for coping with the problem is an ethyl benzene separation column in the isomerizer-separator loop when using catalyst other than those characteristic of Octafining. It will be seen that Octafining does not have this expensive auxiliary to prevent build up of ethyl benzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethyl benzene.
The Octafining process has been extensively discussed in the literature, for example:
1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770 (1955).
2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955.
3. Ciapetta, F. G., U.S. Pat. No. 2,550,531 (1951).
4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189.
5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
A typical charge to the isomerizing reactor (effluent of the crystallizer) may contain 17 wt. % ethyl benzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. % o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A recent development in this art involves the use of a unique class of zeolite catalysts for isomerization of xylenes in a p-xylene recovery loop. The zeolite catalysts designated ZSM-5, ZSM-12 and ZSM-21 as well as other zeolites having like properties will induce extensive disproportionation of ethyl benzene at very low loss of xylene by that reaction, all as described in U.S. Pat. No. 3,856,872, Morrison, dated Dec. 24, 1974. As shown in that patent, isomerization of C.sub.8 aromatics with such zeolite catalysts avoids build-up of ethyl benzene in the loop by converting that compound to lower boiling benzene and higher boiling polyalkyl benzenes which are separated by inexpensive splitters and strippers in the loop.
Another solution to the ethyl benzene problem, in addition to Octafining and the Morrison process, has been to supply xylenes which are free of ethyl benzene. The favored sources of such pure xylene streams are techniques for conversion of toluene as by disproportion and methylation.
Disproportion of toluene can be accomplished with porous acid solid catalysts to yield benzene and a mixture of xylenes. The product is, of course, free of ethyl benzene. See, for example, U.S. Pat. No. 3,578,723, Bowes and Wise, dated May 11, 1971.
Reaction of toluene with a methylating agent such as methanol produces xylenes and higher boiling polymethyl benzenes which are readily separated from the product xylenes and may be reacted with toluene to form additional xylenes by transalkylation reactions. Recent developments in synthesis of xylenes by methylation of toluene have been constituted by provision of catalysts which favor production of p-xylene such that the product xylene streams contains a proportion of p-xylene much in excess of the thermodynamic equilibrium value, thereby facilitating separation of p-xylene at reduced cost. These catalysts having enhanced capability for formation of p-xylene generally manifest a restriction on rate of diffusion of xylenes other than the para isomer, a property conveniently measured as rate of diffusion of o-xylene as set out more fully hereinafter.
Because the characteristic that makes these zeolites selective for p-xylene in methylation of toluene is utilized in the present invention, certain of the patents describing methods for preparation and use of such catalysts are herewith incorporated by this reference as disclosure of methods for preparation of catalysts used in this invention:
______________________________________ 3,965,207 Weinstein June 22, 1976 3,965,208 Butter & Kaeding June 22, 1976 3,965,209 Butter & Young June 22, 1976 3,965,210 Chu June 22, 1976 All in U.S. Classification 260/671 M ______________________________________